1. Field of the Invention
This invention relates to antennae for wireless signal transmission, and more particularly to a low profile cellular antenna design meant for facilitating cellular telephone communications in an inconspicuous manner.
2. Description of the Related Art
Generally, disk or patch antennae have a circular disk or rectangular patch configuration and are elevated above a larger ground plane. The transmission line is connected to the center for the circular disk and at an edge or corner for the rectangular patch to serve as the signal feed. Dielectric material is used to support the elevated portion of the antenna above the ground plane.
These antennae are often, but not always, made using printed circuit board materials and techniques. When used in mobile radio applications, the bottom ground plane disk is made the same size as the upper disk and must be mounted against the metal body of the vehicle. Such disk antennae can be made with major dimensions on the order of one-fifth (1/5) wavelength with a spacing between the top element and the ground element on the order of point zero four (0.04) to point one zero (0.10) wavelength.
The major difficulties with such antennae are:
1) a relatively narrow bandwidth on the order of two percent (2%) to three percent (3%) of the center frequency. Wireless telephone applications generally require bandwidths more on the order of seven percent (7%) to ten percent (10%); PA1 2) a feed geometry that is difficult to adapt for mobile radio applications where the antenna must be mounted to the vehicle with a standardized connector system; PA1 3) A high radiation angle, on the order of sixty degrees (60.degree.) to ninety degrees (90.degree.) above the horizon when they are mounted on a horizontal surface of an automobile or other vehicle; and PA1 4) various manufacturing and fabrication difficulties. The positions of hidden posts must be carefully located. PA1 1) Adding radiating surfaces and increasing the volume of the antenna as shown in the McKenna et al. '538 patent (U.S. Pat. No. 4,835,538 issued to McKenna et al. on May 30, 1989 for a Three Resonator Parasitically Coupled Microstrip Antenna Array Element); PA1 2) Adding an impedance compensating network as shown in the Pues '122 patent (U.S. Pat. No. 4,445,122 issued to Pues on Apr. 24, 1984 for a Broad-Band Microstrip Antenna); PA1 3) Placing selected impedances into the radiating surface. For example, such impedances may be in the form of inductive posts as shown in the Reggia et al. '480 patent (U.S. Pat. No. 4,051,480 issued to Reggia et al. on Sep. 27, 1977 for Conformal Edge Slot Radiators), or in the form of irregularities in the radiating surface (a ninety degree (90.degree.) radial extension of the disk) as shown in the Conroy '976 patent (U.S. Pat. No. 4,160,976 issued to Conroy on Jul. 10, 1979 for a Broadband Microstrip Disc Antenna); and PA1 4) introducing resistances into the radiating surface and thus lower the Q of the antenna.
The operating bandwidth of disk antennae may be increased in one of four known and different ways:
In order to improve the radiation angle, making it lower, a dielectric structure may be used. U.S. Pat. No. 3,414,903 issued to Bartlett et al. on Dec. 3, 1968 for an Antenna System with Dielectric Horn Structure Interposed Between the Source and Lens discloses the use of a dielectric cone to adjust, rather than focus, the radiation pattern of an antenna.
For coaxial antenna connector systems often used on vehicles, a probe feeds the antenna through the mount. Capacitor coupling of the antenna to the feed cable is known in the art as reflected by Alexander, "Capacitive Matching of Microstrip Patch Antennas," IEE Proceedings, Vol. 136, Pt. H, No. 2, April 1989, pp. 172-174. Such capacitive coupling advantageously eliminates the need for tuning posts in such probe fed antennae.
One variation on the patch antenna is known as the Planar Inverted F Antenna or PIFA. Patch and PIFA antennae are compared in Fujimoto and James, Mobile Antenna Systems Handbook, Artech House, Boston, 1994, pp. 160-161. A short theoretical development of the PIFA is included in Fujimoto et al., "Small Antennas," Research Studies Press Ltd., Letchworth, England, pp. 127-131.
The PIFA has been investigated for its superior radiation pattern in mobile telephone operation. One such investigative study is Kuboyama et al., "Experimental Results with Mobile Antenna Having Cross-Polarization Components in Urban and Rural Areas," IEEE Transactions on Vehicular Technology, Vol. 39, No. 2, pp. 150-160.
The antenna shown in FIGS. 1 and 2 is typical of such an antenna and has a bandwidth on the order of two percent (2%) of the center frequency. The edge feeding arrangement does not lend itself as readily to flush mounting as a center feed does. One commercial version of this antenna has the further disadvantage of requiring a grounded tuning wire separate from the sheet metal of the radiator.
From the foregoing, it can be seen that while current disk, patch, or PIFA antennae have some advantages, they nevertheless incur some drawbacks which inhibit or hinder their use in conjunction with cellular telephone transmissions. Consequently, in order to achieve an inconspicuous cellular telephone antenna, it would be advantageous to overcome these disadvantages and provide a wider bandwidth through a disk or patch antenna while maintaining the inconspicuous nature of it. FIGS. 1 and 2 show one embodiment of a current patch antenna with a coaxial cable feeding the antenna from one edge.